This invention relates to exhaust gas turbochargers. More specifically, the present invention relates to variable intake volute geometry for gas turbochargers and control means therefor.
Particularly in the case of small turbines, for example for motor vehicle exhaust gas turbochargers, a volute entry duct is often used before the rotor blading, instead of guide equipment, because a turbine casing of this sort is less expensive to manufacture without guide equipment.
The torque of a normal supercharger turbine with constant, non-adjustable entry duct flow section falls greatly at throughputs beneath the nominal throughput for which the turbine is designed. For a given throughput, the required tangential component of the flow velocity at the wheel inlet is determined by the narrowest flow section at the beginning of the volute part of the inlet duct and its radial position, relative to the inlet diameter of the rotor. In order to achieve acceptable turbine powers in the partial load range with varying exhaust flow, proposals have been made to alter these geometrical data. In general, these proposals consist of altering the volume of the inlet volute space by movable walls to correspond to the variable exhaust gas flow.
The ASME publication "Aerodynamic Design of Fixed and Variable Geometry Nozzleless Turbine Casings" describes the possibility of altering the inlet volute space by an axially movable wall corresponding to the exhaust gas throughput. This involves the acceptance of a clearance relative to the casing inner wall. This solution is shown only schematically in the paper without any information about the adjustment equipment and the design arrangement.
Another very complex design solution with large leakage flow losses is described in the U.S. Pat. No. 2,916,198. The volume of the effective inlet volute space can, in this case, be altered by two mutually opposed half moon shaped flaps which can be swivelled about axes lying approximately tangentially to the volute periphery. The operation of the flaps is carried out by a rod situated outside the casing. It is not possible by this structure to use the exhaust gas pressure in a simple manner to adjust the flaps.
The German Offenlegungsschrift No. 2,934,041 describes an appliance for solving the present task in which the intake in the volute space is subdivided into several flutes whose cross sections, inter alia, can be altered by walls movable in the axial direction from outside. Variants of this appliance have closing elements in these flutes and the degree of opening of these elements can be adapted to the exhaust gas flow. These solutions are not satisfactory in the case of partial opening of the closing elements.
The present invention, in which at least one wall element of an exhaust gas turbine is movable along a rotor shaft axis in a cylindrical part of an inlet volute, arose from the object of creating an embodiment of a controllable turbocharger which is usable in practice and avoids the disadvantages of known solutions.